This invention relates to an acidic aqueous fountain etch concentrate solutions useful for lithographic printing. Lithographic printing is a well known and established art. In general, the process involves printing from a flat plate or cylinder, usually anodized aluminum, having substantially no surface relief and depends upon different properties of the image and non-image areas of the surface for printability. In lithography, the image to be reproduced is imparted to the plate by any one of several methods well known to those skilled in the art in such a way that the non-image areas are rendered hydrophilic while the image areas are hydrophobic. A widely practiced technique employs a photosensitive coating for this purpose. Following exposure of the photosensitive coating to imagewise modulated light, the latent image is developed and a portion of the coating is removed from the plate. Next, the plate is treated with a desensitizing solution (so called finisher or preserver composition) to render the plate hydrophilic in the areas from which the photosensitive coating has been removed. During the actual printing process, an aqueous fountain solution is applied to the plate surface. The fountain solution keeps moist all portions of the surface not covered by the hydrophobic image. Furthermore, the fountain solution prevents the plate from scumming, i.e., it prevents the non-image areas from becoming at least partially ink receptive. The fountain solution may be formulated to gradually etch the surface of the plate just enough to keep the lines sharp and prevent rapid wear. In a conventional system, the fountain solution is applied to the plate by one or more rollers. At least one ink roller coated with an oil based printing ink then contacts the entire surface of the plate but deposits the lithographic ink only on the image area since the hydrophilic non-image areas repel the ink. Hence, for each impression made during a run, the lithographic plate is first dampened with the aqueous fountain solution and then inked with a lithographic ink. Alternatively, the fountain solution and at least a portion of the oil based ink are applied to the plate simultaneously with a form roller. In this latter system, other rollers, usually smaller in diameter than the first, may contact the plate subsequently to distribute the ink more evenly. Finally, the ink on the image is transferred directly to a paper sheet or other receptive surface to be printed, or to an offset blanket of rubber or synthetic material which in turn transfers the print to the final copy surface.
Fountain solutions may be made up by the printer from a fountain etch concentrate by adding additional water and optionally about 10 to 30 percent by volume of isopropyl alcohol. Aqueous acidic fountain etch concentrates of the prior are generally contain a water soluble resin or gum such as gum arabic or cellulose gum and may also contain an etching agent, buffering agents and wetting agents.
Acidic fountain solutions are generally formulated to achieve a variety of functions. Hydrophilic colloids such as gum arabic are employed to keep the non-image areas of the plate hydrophilic during the press run. Acid etchants may also be included to keep the non-imaged areas more sharply defined and ink repellent. Surfactants or combinations of surfactants and alcohols such as isopropyl alcohol are used to lower the surface tension of the solution to permit enhanced wetting of the plate and roller surfaces. This characteristic may be also achieved without using surfactants by using amounts of hydrophilic colloids in excess of that required to maintain hydrophilicity of non-image plate surfaces in combination with one or more metal salts such as nitrates, chromates and the like, and/or alcohols or glycols.
The use of surfactants in such compositions can lead to problems such as foaming or emulsification of the printing ink during the press run. Fountain solutions containing surfactants also tend to wet the paper being printed more than solutions without surfactants, particularly in color applications. This causes the paper to become spongelike and the printing ink to diffuse somewhat on the paper, leading to less than sharp images. The use of higher than necessary levels of colloid can lead to scumming or blinding of the plate and can also enhance the tendency of the printing ink to emulsify. The use of salts such as nitrates with excessive colloid (gum arabic) can further complicate the blinding problem due to the tendency of these materials to complex with gum arabic yielding gum arabic salts which usually cause plate blinding. The use of significant quantities of alcohols such as isopropyl alcohol in fountain solutions tends to reduce the solubility of other components present in the solution which can then build up on roller or plate surfaces as they leave solution. In addition, isopropyl alcohol is undesireable because it is toxic, flammable, and has a low vapor pressure which leads to rapid evaporation.
Variations in such formulations have been disclosed in the prior art. For example, U.S. Pat. No. 4,266,481 teaches the replacement of gum arabic in fountain solutions with a composition comprising a polyacrylamide polymer to avoid, inter alia, the tendency of gum arabic based fountain solutions to emulsify the ink used in the lithographic process, thereby weakening the printing resolution and contributing to scumming of the plate and stripping of the ink from the ink rollers. The tendency of gum arabic to lead to plate blinding (a condition where it deposits onto the image bearing surface of the plate rendering it partially hydrophilic) is recognized by U.S. Pat. No. 4,400,481 in aqueous based plate finisher and preserver compositions. The patent teaches that this problem can be avoided in such formulations by utilizing a three component mixture comprising gum arabic, polyvinylpyrrolidone and polyacrylamide. However finisher compositions as disclosed in this patent would not be easily adapted for use in a fountain solution because of a tendency to cause ink emulsification and transfer problems and excess buildup of solids on lithographic dampening roller systems.
There is a persistent demand, for improved fountain solutions for offset printing which not only minimize scumming or blinding of the printing plates, but also offer other advantages such as uniform pH control, resistance to buildup of solution residues on transfer rolls or roll covers, improved print quality, reduced tendency to wet the printing paper, faster ink drying and improved ink/water balance.